Earlier U.S. Pat. No. 4,116,145 as well as commonly owned U.S. Pat. No. 3,196,815 disclose a sewing machine having separate feed and holddown feet. These feet lie immediately adjacent each other and are oppositely vertically displaced. The feed foot can also be horizontally reciprocated. These two feet coact with a lower feed foot or dog provided under the sewing-machine stitch plate and jointly horizontally reciprocated with the upper feed foot.
The upper feed foot normally is brought down onto the goods lying on the lower feed foot and once the goods are pinched between these feed feet the holddown foot is lifted. Then the two feed feet move a step forward to advance the goods an increment, after which the holddown foot is dropped to pinch the thus-advanced goods against the stitch plate. The upper and lower feed feet are then respectively raised and lowered, then moved one step back to their starting positions, so that they can again pinch the goods and advance them one more increment. In this manner positive advance is insured even at relatively high speed.
My earlier above-cited U.S. Pat. No. 4,116,145 converts the rotary motion of the main upper drive shaft of the machine into reciprocating vertical motion of these feet by means of a crank disk connected to a bell crank via an input crank. The bell crank in turn swings a composite connecting rod comprising a sliding link which interconnects the bell crank with a ternary link carrying the feet and comprising a guide bar for guiding the sliding link. The ternary link is urged by a spring toward the stitch plate by a swingable arm and alternatingly lifts the pressure and feed feet. The velocity of these feet is reduced sinusoidally toward zero as they come into engagement with the material being sewn.
This system described immediately above is an improvement on the earlier system described in the other above-cited U.S. Pat. No. 3,196,815. In that arrangement the velocity of the feet is at a maximum when they come to their lower end positions in which they contact the goods. Reducing the velocity to zero as the feet come into engagement with the goods reduces the tendency of the feet and goods to bounce and flutter, especially in high-speed work. In addition the system of my earlier patent eliminates the necessity of providing dampers for the feet to absorb some of the energy of the contact blow.
In the known arrangements the ternary link carrying the two feet is urged downwardly by a compression spring whose compression can be adjusted to vary the foot pressure in a manner well known in the art. This spring also compensates for the various forces which are created in the linkage especially when working at high speed. When a strong spring is used there is the danger of damaging the goods being sewn; when a weak spring is used vibration or flutter is likely. In addition a strong spring exposes the various pivots to considerable wear and generally makes the machine work under a heavier load than is strictly necessary for the job at hand.